Add onions to the bottom of a 6-8 quart slow cooker. Mix together salt, pepper, garlic and thyme and season all sides of the roast by gently sprinkling spices onto the outside.
Place the roast, fat side up, into the slow cooker on top of the onions. Slowly stir in beef broth around the roast. Cover and heat on high for 6 hours or low for 10 hours.
To Make the Gravy: Carefully remove 2 cups of beef drippings from the slow cooker and add to a saucepan over medium high heat. Stir in worcestershire sauce and onion powder until mixture begins to boil. Whisk together cornstarch and water in a dish until smooth and slowly stir in, boil until thicker. Remove from heat and serve gravy on top of sliced roast. Top with chopped fresh parsley if desired.